To obtain hydrocarbons such as oil and gas from subterranean formations, wellbores are drilled into the formations by rotating a drill bit attached to an end of a drill string. A substantial portion of current drilling activity involves what is referred to in the art as “directional” drilling. Directional drilling involves drilling deviated (e.g., horizontal) wellbores, as opposed to straight, vertical wellbores. Modern directional drilling systems conventionally employ a bottom hole assembly at the end of the drill string that includes a drill bit and a downhole motor to drive rotation of the drill bit. The drill bit is coupled to a drive shaft of the downhole motor, and drilling fluid pumped through the motor to the drill bit from the surface drives rotation of the drive shaft to which the drill bit is attached. Such downhole motors are commonly referred to in the drilling industry as “mud motors,” “hydraulic drilling motors,” and “Moineau motors.”
Downhole motors include a power section that contains a stator and a rotor located within the stator. The stator may include a tubular metal housing having a helically contoured or lobed elastomeric material (e.g., a rubber liner) on walls of an internal bore defined by the housing. The rotor is usually made from a suitable metal, such as steel, and has an outer lobed surface. Pressurized drilling fluid (e.g., drilling “mud”) laden with a substantial volume of solids is pumped into a progressive cavity formed between the rotor and the stator lobes. The force of the pressurized fluid pumped into and through the cavity causes the rotor to rotate. The shaft of the rotor is coupled to a bearing assembly having a drive shaft (e.g., a “drive sub”), which in turn rotates the drill bit attached to the drive shaft.
As drilling fluid flows through the progressive cavity between the rotor and the stator, the drilling fluid may abrade and erode surfaces of the rotor and the stator within the progressive cavity. Such abrasion and erosion may be relatively more severe at locations at which the direction of fluid flow changes, since the drilling fluid and particularly the solids suspended therein, may impinge on the surfaces at relatively higher angles at such locations. Removal of material due to abrasion and erosion can eventually result in the alteration of the profile of lobes of the rotor as well as the profile of interior surfaces of the stator, which can adversely affect operation of the hydraulic drilling motor. In addition, the high downhole temperatures experienced during drilling may degrade the materials of downhole motors, particularly the elastomeric material located on the walls defining the inner bore of the stator. When the elastomeric materials degrade, portions of the stator and rotor that are intended to seal against one another may leak and permit drilling fluid to bypass the intended confined flow path between the stator and rotor, which reduces the efficiency and power of the downhole motor for a given flow rate.